1. Field of the Invention
The invention relates generally to electronic aids to air navigation, and more particularly, to the so-called scanning-beam microwave landing system.
2. Description of the Prior Art
The known conventional time-reference scanning-beam microwave landing system includes transmitting apparatus and a mechanically or electrically scanned antenna arranged to scan a single frequency, collimated beam throughout an angular sector of interest. The time of reception of the beam with respect to a reference pulse gives angular information, and the detected signal at the remote receiving station (such as for example, in the aircraft on landing approach) of the system is related directly to the received power radiating from the scanning beam antenna and as such, it is incoherent or non-coherent in nature. The use of coherent signal processing techniques, which may be required or highly desirable to achieve the required accuracies in difficult situations, for example, in respect to low angle coverage in the elevation plane, is not possible. Coherent detection is usually associated with "two-way" radar systems, and in such devices it is possible to use means to "remember" (store) the transmitted phase while waiting for an echo signal which may then be processed against the remembered (stored) transmitted phase. One way of "remembering" the transmitted phase in such 2 way arrangements is to heterodyne-down the microwave transmitted signal and use it to lock-in a coherent oscillator (coho) usually in the IF domain. Received echo signals are then reduced to the IF domain and are coherently (phase) detected.
The text Radar Handbook by Merrill I. Skolnik (McGraw Hill 1970), particularly Chapter 17 thereof, provides a background in connection with coherent detection, FIG. 3 of that chapter outlining the concept in a straightforward prior art radar system.
Since the so-called scanning-beam landing system is a "one-way" system for providing air-derived angle data using the ground scanning transmission as a beacon, the usual technique of simply "remembering" the transmitted phase, while waiting for a processable echo signal, is not available.
The transmitted beam mechanical scanning techniques and also the electronic (inertialess) scanning techniques (such as in connection with phased-array scanning) are prior art, per se, as they may be incorporated in the combination of the present invention. The aforementioned Radar Handbook test is also a useful reference in connection with the various known forms of beam scanning, as may be employed herewith.
The manner in which the present invention provides the capability for coherent airborne signal processing will be understood as this description proceeds.